NEW YORK (CBSNewYork) — Mercury levels have been testing higher than ever in certain fish and are increasing, prompting doctors to reemphasize their advice that pregnant women and young children should avoid eating them.

As CBS 2’s Dr. Max Gomez reported Monday, mercury in fish actually comes from coal-burning power plants. All the coal contains tiny amounts of mercury, but it adds up when millions and millions of tons of coal are burned each year.

Emissions from the coal-burning power plants release the mercury into the atmosphere, which can travel thousands of miles before coming back to the Earth or the ocean.

Billions of tons of coal being burned in Asia – especially India and China – have sent all the mercury over the Pacific Ocean. That is where it gets into the ocean’s food chain.

“The microorganisms – the bacteria — in the sea convert this metallic mercury, which is not terribly hazardous, into another form of mercury called methyl mercury, which is very hazardous,” said Dr. Philip Landrigan, Dean for Global Health at Mount Sinai Medical Center.

Now, a new study in the journal Nature Geoscience details what then happens to the toxic mercury. Plankton eat the mercury, which are then eaten by small fish, and up the food chain.

“I know that the larger the fish, the higher the mercury content, because the big fish eat the little fish,” said Julie McDermott of Manhattan.

And that is the problem. The mercury is bio-concentrated as it moves up the food chain, until it reaches hazardous concentrations in large predator fish.

Eating more than a little of such fish is not recommended for pregnant women and very young children.

“In the baby’s brain, it kills neurons – it erodes the connections between cells – and the result is the babies are born with three or five or eight points lower IQ than they should have had,” Landrigan said. “They tend to have shorter attention spans, and behavior spans.”

The fish with the highest mercury content include shark, swordfish, king mackerel, and tilefish.

Those with the lowest amounts are shrimp, canned light tuna, salmon, pollock and catfish.

The message reached everyone who spoke to CBS 2 on Monday.

“I’ve been trying to stay away from it a little more – especially the bigger fish,” said Jordan Nadler of Manhattan.

Experts emphasized that fish is still good for you – including pregnant women – as long as it is low mercury content fish.

For the long term, the study called for global controls on coal burning to reduce mercury in the environment.